"Thermoreversible gel" is a gel crosslinked by bonds of the physical type, capable of being transformed by heating within a comparatively narrow range of temperatures to a sol, i.e. a solution of the polymer. Upon cooling below the temperature of gelation, the liquid sol solidifies again to gel.
As solvents there are used liquids capable of dissolving polyacrylonitrile and its copolymers, either alone or blended to form a co-solvent mixture. Examples are dimethyl formamide, dimethyl sulfoxide, cyclic ethylene carbonate, nitric acid, sufficiently concentrated aqueous solutions of sodium or calcium rhodanide or of zinc chloride. In the case of copolymers, particularly so called multiblock-copolymers, the solvent must be capable of dissolving also the other component, i.e. not only polyacrylonitrile, but also e.g. polyacrylamide forming the sequences alternating with polyacrylonitrile.
Acrylonitrile polymers or copolymers useful in the present process are preferably those displaying in the absence of solvents of polyacrylonitrile the typical X-ray pattern with a 5.1 A periodicity, the reflexes appearing in equatorial position upon stretching the sample. The crystallograhic structure of polyacrylonitrile is described e.g. in the paper of C. K. Bohn, J. R. Schaefgen and W. O. Statton in J. Polymer Sci. 55, 531 (1961). The structural features of polyacrylonitrile remain preserved in copolymers--so called block copolymers--containing sufficiently long sequences of acrylonitrile units. Random copolymers, however, have to contain at least 70% of acrylonitrile units to show the said typical X-ray pattern, while multi-block copolymers display it even if they contain but 1% of acrylonitrile units, due to the sequences in which the units are arranged.
A typical feature of such multi-block copolymers is their solubility in solvents capable of dissolving polyacrylonitrile, or in solvent mixtures containing such solvents. Even the above mentioned multi-block copolymers containing a very low proportion of acrylonitrile units, arranged, however, in more or less long sequences, are soluble only in solvents which are typical solvents of pure polyacrylonitrile, e.g. dimethyl sulfoxide. Other known solvents of polyacrylonitrile which can be used here are: Dimethyl acetamide, dimethyl methoxyacetamide, N-formyl morpholine, N-formyl hexamethylene imine, cyclic tetramethylene sulfone, 1,2,3-trithiocyano propane, gamma-thiocyanobutyronitrile, some cyclic lactones and lactams, carboxylic acids such as formic and halogenacetic acids, nitric acid with a concentration higher than 40%, 70-85% sulfuric acid, hydrofluoric acid, highly concentrated phosphoric acid, concentrated aqueous solutions of alkali metal perchlorates or of lithium bromide, and other solvents mentioned above. The list is not complete and the solvents are mentioned by way of example only, other solvents of polyacrylonitrile being not excluded.
The precipitants are such as to be incapable of dissolving polyacrylonitrile. They can be, however, solvents or swelling agents for the other component or components of the copolymer. Typical precipitants, miscible with the said solvents, are water, lower aliphatic alcohols, glycerol, ethylene glycol, polyethylene glycols such as di(ethyleneglycol), monoethers and monoesters of aliphatic glycols, and many others. For the most important multi-block copolymers of acrylonitrile with acrylamide or with acrylic acid, respectively, water is not only the cheapest but also the best precipitant in many respects, particularly if the shaped articles are designed for use in surgery.
In the case of solvents consisting of more than one component such as inorganic aqueous acids or aqueous salt solutions, the solution with the minimum concentration capable of dissolving polyacrylonitrile is considered the solvent, the surplus of water, if present, being the precipitant. The minimum concentration is either known or can be easily determined. It depends on the molecular weight and temperature.